1. Field of the Invention
The present invention relates to a solution casting process for producing polymer film. More particularly, the present invention relates to a solution casting process in which polymer film can be produced at a high efficiency and stability.
2. Description Related to the Prior Art
Cellulose acylate film as polymer film, for example cellulose triacetate (TAC) film, is produced by a solution casting process, and widely used in a liquid crystal display panel, photosensitive material, and the like. Today, the liquid crystal display panel has been marketed more and more widely. So the production of the polymer film is required to have higher efficiency. For example, a casting speed of the solution casting process is desired to be higher. A thickness of the polymer film is desired to be smaller. To obtain the TAC film as the polymer film, at first the cellulose triacetate is dissolved in a mixed solvent of which a main component is dichloro methane, to prepare polymer solution or dope. The dope is cast on a support, to form gel film. The gel film is dried on the support to come to obtain a self-supporting characteristic, before the gel film is stripped from the support to obtain the polymer film. This is a widely known method of the solution casting process.
Due to social requirement, it is strongly suggested to reduce or suppress the use of dichloro methane as chlorine-containing solvent in particular in consideration of protection of environment. One suitable substitute is a mixed solvent including methyl acetate as a main composition because of considerably low influence to the environment. However, comparison according to experiments has resulted in somewhat difficulty in strengthening the cooled gel if the dope is prepared from the mixed solvent of the methyl acetate type in comparison with the dope prepared from the dichloro methane solvent. There arises a problem in that the gel film does not have a sufficient self-supporting characteristic even by casting of the dope on the support. JP-A 10-045917 and 2001-294667 suggest the solution casting process according to casting in a cooled manner in which the support is cooled to promote gelling of the gel film cast on the support, and stripping of the gel film is facilitated.
However, the casting process in the cooled manner still has a problem in a reduced strength of the polymer film sufficient for the stripping after the imparting of the self-supporting characteristic to the gel film according to casting and gelling on the support. This is typically because of rise in the casting speed and the reduction of the thickness of the polymer film. As the insufficiency occur in the strength of the polymer film upon stripping, non-stripped remainder of the gel film occurs on a surface of the support. Furthermore, an upper shift of a stripping line occurs to influence stability in the feeding. Also, dew condensation is likely to occur on the support due to forced cooling of the support. Gasified solvent in the air is changed to liquid again, and also deposits the support due to the cooling. Those problems are harmful to continuous operation for casting.